1. Field of the Invention
The present invention relates to a method for manufacturing honeycomb structures made of titanium, and in particular for manufacturing honeycomb structures consisting of a planar assembly of a number of hollow columnar cells with titanium or a titanium alloy used as base material.
2. Description of the Related Art
Honeycomb cores consisting of a planar assembly of a number of hollow columnar cells have a variety of excellent characteristics such as high ratios of strength to weight, and are widely used as various of construction materials. Although metals, plastics, papers, or the like are used as the base material for these honeycomb cores depending on their use, typical metals are aluminum (aluminum alloy) and stainless steel.
Conventionally, the honeycomb cores are manufactured in the corrugation or expansion process. And, bonding in a striped pattern between cell walls of the honeycomb cores are performed by 1 spot welding, 2 adhesion, 3 brazing.
In the corrugation process, after very thin foil-like base material is formed (corrugation) into corrugated sheets, a number of the corrugated sheets such formed are piled together in such a manner that the crests of a corrugated sheet and the troughs of the next corrugated sheet meet together with a half pitch shift between them. Each joint between the crests and troughs are bonded by 1 using a spot welding device, 2 adhesion with heating under pressure, 3 brazing a brazing material, or the like. Hereby, the honeycomb core having the cell walls made of the corrugated sheets piled, and consisting of the planar assembly of the number of hollow columnar cells that are separated by the cell walls, has been manufactured.
On the other hand, in the expansion process, after 2 adhesive or 3 brazing material is applied in the striped pattern on the surfaces of very thin foil-like base material, a number of the base material sheets are piled with a half pitch shifting between them. Thereafter, the base material is bonded in a striped pattern with heating and pressing for 2 adhesion or 3 brazing, and the base materials piled are elongated by tensile force in direction of the pile. Thus, the honeycomb cores having the elongated base material as cell wall, and consisting of the planar assembly of the number of hollow columnar cells that are separated by the cell wall, have been thus manufactured.
In the fields where honeycomb cores made of metal are used as structure materials, namely, honeycomb cores having base material of metal such as aluminum, aluminum alloy, stainless steel, or the like are used as structure materials, for example, in the fields for manufacturing structure materials and parts of aircraft, it is desired for honeycomb cores to become more and more light and stiff, and have higher heat resistance, corrosion resistance, and the like.
Then, in the field for manufacturing aircraft and the like, as materials having these high excellent characteristic, in stead of aluminum (aluminum alloy) and stainless steel, the use of titanium (a titanium alloy) has been progressed.
However, in spite of these backgrounds, regarding to the honeycomb structure having base materials of titanium or a titanium alloy (hereinafter abbreviated simply as "titanium honeycomb"), some problems described below are pointed out in the manufacturing process.
Concerning the method for manufacturing titanium honeycomb, some problems as follows have been pointed out, conventionally.
1. Although the titanium honeycomb has been conventionally manufactured by the corrugation process, in the corrugation process, complex process and high production cost have been pointed out, because piling and bonding were performed after the corrugated sheets have been formed in this process.
Further, large space has been required from first to end of the process after all, because formation of corrugated sheets requiring large space is performed in the first step before piling and bonding. Thus, it has been pointed out that large-sized titanium honeycomb or titanium honeycomb having many cells are manufactured with difficulty. That is to say, it has been difficult to manufacture a large block of titanium honeycomb or the honeycomb structure having small-sized cells by the use of the corrugation process.
In this way, concerning the conventional method for manufacturing titanium honeycomb, with employing the corrugation process, high production cost and difficulty of manufacturing the large-sized titanium honeycomb are pointed out. (These problems are generally pointed out not only in the manufacturing method of the titanium honeycomb, but also in a method for manufacturing honeycomb cores by the corrugation process.)
2. When titanium honeycomb is manufactured by the corrugation process, each joint between the corrugated sheets (the cell walls) piled are bonded by 1 spot welding, 2 adhesion, 3 brazing or the like as mentioned above.
And, first of all, in case of 1 spot welding, it has been pointed out that the production cost becomes high because a number of spots have to be welded.
Furthermore, in the case of 2 adhesion, it has been pointed out that the characteristics of the titanium honeycomb manufactured, such as stiffness, heat resistance, corrosion resistance, or the like are determined by the characteristics of the adhesive used. In other words, the difficulty is that the essential excellent characteristics of titanium or titanium alloy used as base material, such as stiffness, heat resistance, corrosion resistance, or the like, are not utilized.
Furthermore, in case of 3 brazing, occurrence of erosion, generation of intermetallic compounds, and inclusion of undesirable metals are pointed out as problems.
That is, the base materials of titanium or titanium alloy (corrugated sheets and cell walls) are corroded with brazing material by heat or pressure in brazing, and as a result, erosion which means formation of holes and exfoliation on the surfaces occurred easily. Further, it is easy to generate rigid and brittle intermetallic compounds between the base materials of titanium or titanium alloy (the corrugated sheets) and brazing material, as results of the reaction and fusion. In addition, difficulty occurs in corrosion resistance of the titanium honeycomb, with the resulting that the joints between the respective base materials of titanium or titanium alloy (the corrugated sheets and the cell walls) are bonded with the brazing material having the metals which are different from the metal of base material.
Therefore, in case of brazing, difficulties occur in the properties of the produced titanium honeycomb, such as stiffness, corrosion resistance, or the like. Therefore, the difficulty is that the excellent identities of titanium or a titanium alloy, for example, stiffness, corrosion resistance, or the like, are not utilized as base materials.
As described above, concerning the conventional methods for manufacturing titanium honeycomb, problems have been pointed out in production cost, stiffness, heat resistance, corrosion resistance, or the like, because the joint in respective base materials are performed by 1 spot welding, 2 bonding, and 3 brazing.
3. Considering the first problem described above, the expansion process has been employed instead of the corrugation process. Further, considering the second problem described above, the methods for manufacturing titanium honeycomb in which the diffusion bonding is performed for joining between respective base materials without 1 spot welding, 2 bonding, or 3 brazing, have been developed recently. Inventors and an applicant of the present invention have been invented such a method and filed this invention as Japanese Patent Application No.9-326912.
In the method employing the expansion process for manufacturing titanium honeycomb, first, parting agent is applied in a striped pattern to each of the foil-like base material of titanium or titanium alloy with regular widths and pitches. Before the direct diffusion bonding between respective base materials of titanium or a titanium alloy are performed in a striped pattern, it is necessary to apply the parting agent to no bonding portion other than bonding portion where the diffusion bonding is performed.
As the parting agent, L-hexagonal boron nitride is conventionally used in general processes. That is, when general metals are processed by heat treatments such as brazing or sintering, the parting agent containing L-hexagonal boron nitride is widely used. Because when the parting agent is applied to surfaces of metal instruments such as metal plate, jig, metal mold, or the like, it gives excellent parting effect, such parting agent containing L-hexagonal boron nitride is used in the methods for manufacturing titanium honeycomb.
However, when the parting agent containing L-hexagonal boron nitride is used for heat treatment of titanium or a titanium alloy as active metals, the parting agent loses the excellent parting effect, because brittle layers are made by the result of reaction between titanium or a titanium alloy and L-hexagonal boron nitride.
That is, in the method for manufacturing titanium honeycomb by the use of expansion process utilizing the diffusion bonding, when the parting agent containing L-hexagonal boron nitride is applied in a striped pattern on the base material of titanium or titanium alloy, a reaction occurs between surfaces of titanium or titanium alloy which is an active metal having high activity at high temperature, and L-hexagonal boron nitride contained in the parting agent applied. Hereby, hard and brittle intermetallic compounds such as titanium nitride, titanium boride, and the like are generated.
In this manner, brittle layer is formed on the surface of titanium or a titanium alloy as base material, and the brittle layer cracks. When the reaction further proceeds, the prescribed parting effect of the agent is lost.
For example, the parting agent containing hexagonal boron nitride (L-BN) is applied on the surface of foil-like titanium or titanium alloy, and heat treatment is performed. Thereafter, the temperature of the treated matter is returned to room temperature and tensile test is performed. As a result, breaking elongation point becomes much lower than that of the titanium or titanium alloy which is similarly treated except for applying the parting agent. The lowering of elongation is caused by the formation of the hard and brittle intermetallic compounds described above, namely the formation of the brittle layers.
In the method for manufacturing honeycomb cores made of titanium by employing expansion process in which bonding is performed by the diffusion bonding, when the parting agent containing hexagonal boron nitride (L-BN) is applied on the surfaces of foil-like titanium or a titanium alloy, reactions are occurred on the surface by the heat treatment for diffusion bonding. As results, the brittle layers are formed on the surfaces of titanium or a titanium alloy, elongation at the breaking point of the treated metals becomes low, and the prescribed parting effect of the agent is lost.
In the expansion process, the brittle layers formed on the base material cause breaking of titanium or titanium alloy as base materials piled. Further, insufficient expansion of the base materials makes elongation of the base materials made of titanium or titanium alloy and piled difficult. Eventually, when the prescribed parting effect of the agent is lost, elongation in the expansion process becomes impossible because respective base materials of titanium or titanium alloy are joined uniformly with each other by diffusion bonding.
Although method for manufacturing titanium honeycomb by employing expansion process in which diffusion bonding is used has been developed recently, the use of this method has not progressed. Because it is pointed out that parting agent applied for no bonding portion causes difficulty or impossibility in expanding the base materials made of titanium or titanium alloy (for corrugated sheet and cell walls).
Considering these matters, the method of the present invention for manufacturing the titanium honeycomb has been invented in order to solve the problems of the above-mentioned prior art which includes the method has been developed recently. The titanium honeycomb is manufactured through the steps of using the titanium or titanium alloy as base material, performing diffusion bonding in a striped pattern between these materials by heat treatment with pressing, and elongating these base materials. Further, powder of rare earth oxide such as yttrium oxide is employed as parting agent. The powder is mixed with, for example, the organic binder having 30 .mu.m or less particle size and vaporizing at less than 350.degree. C. in an amount of 1% by weight or more. This parting agent is applied in the thickness of 0.3-30 .mu.m.
Therefore, the object of the present invention is to provide the method for manufacturing the titanium honeycomb having advantages as follows. First, the manufacturing cost of using the present invention is low so that large typed honeycomb structure can be manufactured. Second, the titanium honeycomb manufactured by the method of the present invention has high stiffness, heat resistance, and corrosion resistance because the features of the base material of titanium or a titanium alloy is made the best of. Third, brittle layers of the titanium honeycomb are not formed, parting effect of the parting agent applied is not lost, and, accidents in which a part of the titanium honeycomb is braked and difficulty or impossibility of expanding the titanium honeycomb are not happened, because the parting agent applied dose not react with the base materials of titanium or titanium alloy at the heat treatment.